One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition (“CVD”). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. Plasma-enhanced CVD (“PECVD”) techniques, on the other hand, promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared to conventional thermal CVD processes. These advantages are further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive.
As semiconductor device geometries have decreased in size over the years, the ratio of the height of such gaps to their width, the so-called “aspect ratio,” has increased dramatically. Gaps having a combination of a high aspect ratio and a small width present a challenge for semiconductor manufacturers to fill completely. In short, the challenge usually is to prevent the deposited film from growing in a manner that closes off the gap before it is filled. Failure to fill the gap completely results in the formation of voids in the deposited layer, which may adversely affect device operation, such as by trapping undesirable impurities.
One process that the semiconductor industry has developed to improve gapfill capability uses a multistep deposition and etching process. Such a process is often referred to as a deposition/etch/deposition (“dep/etch/dep”) process. Such dep/etch/dep processes divide the deposition of the gapfill layer into two or more steps separated by a plasma etch step. The plasma etch step etches the upper corners of the first deposited film more than the film portion deposited on the sidewall and lower portion of the gap, thereby enabling the subsequent deposition step to fill the gap without prematurely closing it off Typically, dep/etch/dep processes can be used to fill higher-aspect-ratio small-width gaps than a standard deposition step for the particular chemistry would allow.
Notably, while there has been some recognition that in situ etch processes may be desirable for dep/etch/dep processes, in practice attempts at such processes have been constrained by limitations in the effectiveness of plasma processing systems. For example, one type of plasma processing chamber places a wafer on an electrode of the plasma circuit, opposite another planar electrode, and capacitively couples high-frequency electrical power to the two electrodes to form a plasma between them. Such a plasma reactor has advantages where it is desirable to form the plasma in the presence of the substrate, such as when the physical movement of plasma species to and from the substrate is desired. However, the bombardment by plasma species that results from this type of plasma formation may be undesirable for dep/etch/dep processes.
Another approach to plasma processing generates plasma in a remote location, and couples the plasma to a processing chamber. Various types of plasma generators have been developed, including magnetron sources coupled to a cavity, inductively coupled toroidal sources, microwave irradiation directed at a plasma precursor, electron-cyclotron resonance generators, and others. Remote plasma techniques offer a number of advantages for certain types of processes, such as cleaning, but generally the atomic species that eventually reach the chamber are of relatively low density because of recombination effects. With dep/etch/dep processes, this may result in nonuniformities since the gas distributions differ for the deposition and etching phases.
Inductively coupled plasma systems have been developed that can generate a high-density plasma locally above a wafer, but shield the wafer from the more deleterious effects of the plasma generation by using the plasma itself as a buffer between the wafer and the plasma generation region. These systems typically rely on diffusion of plasma to provide a uniform density across the wafer surface. In one system, a dielectric dome, or chamber top, has a conductive coil would around the dome. High-frequency electric energy provided to the coil couples to a plasma precursor gas in the chamber and converts the precursor to plasma. The fields generated by the coil through the dome have an electric field component normal to the surface of the dome that causes plasma species to be directed to and from the inner surface of the dome. This field component acting on the plasma can cause physical erosion of the inside of the dome, as well as affect the power coupling to the plasma to cause a nonuniform plasma density. The possibility of damage to the dome is further increased during the etch phase of a dep/etch/dep process if the etch phase is performed in situ. This is because etchant species react chemically with the dome material, in additional to the physical bombardment of the ionic etchant species.
It is, therefore, desirable to provide improved methods of performing dep/etch/dep processes that avoid the surface erosion problems of conventional systems while taking greater advantage of the benefits of having all phases of the dep/etch/dep processes performed in situ.